Cell-to-cell interactions (between oral bacteria) are most likely mediated by complementary surface components composed of a lectin on one cell type and a carbohydrate receptor on the other cell type. All of the coaggregations examined to date between oral actinomycetes (Actinomyces viscosus and A. naeslundii) and oral steptococci (Streptococcus sanguis. S. mitis, S. MG-intermedins, and S.moebillourum) exhibit similar properties in that one cell type is resistant and the other is inactivated by heat or protease treatment. Many of these coaggregating pairs are inhibited by lactose, but others are unaffected by this sugar. Coaggregation-defective (COG-) mutants that exhibit a single kind of surface structure that mediates coaggregation were used to probe some of these lactose-insensitive coaggregations. N-acetylneur-aminic acid (sialic acid) inhibited a specific pair consisting of an A. naeslundii and a COG- S. snaguis strain. The effect of human saliva on coaggregation properties of cells was also determined. Only minor differences in coaggregation properties of cells suspended under the two conditions were found.